Connected to discharge lamps, in particular high pressure discharge lamps, for the purpose of ignition is an ignition unit that produces one or a sequence of high voltage pulses in order to ignite the discharge lamp. The high voltage pulses must have a certain minimum ignition voltage Uz for successful ignition.
A high voltage probe and an oscilloscope are normally used to measure the high voltage pulses. FIG. 1 shows the time dependent voltage UL(t) generated at a 400 W sodium vapor high pressure lamp having a first ignition unit. The ignition voltage UZ is the maximum value of the voltage (UZ=3.55 kV). FIG. 2a illustrates the current profile and the voltage profile of the discharge lamp, and the accumulated energy introduced in a long time period. The time-dependent voltage UL(t) and the evaluated ignition voltage UZ (UZ=3.96 kV) are shown in FIG. 2b for a second ignition unit. The ignition voltage can also be measured with the aid of a peak voltage detector in a simple and cost effective way. A peak voltage detector for the evaluation of the positive maximum value of the voltage, is shown in FIG. 3. An analog-to-digital converter is used for the measurement acquisition. If the peak values of each pulse are to be measured for repeated pulses, it is therefore necessary to discharge the capacitor again between the individual pulses, and this can be done, for example, by a resistor of high resistance. The measurement acquisition system must additionally have a sufficiently high acquisition rate. It is possible to use the signals of the peak voltage detector for the purpose of controlling an ignition device.
The energy that is coupled into the discharge during the voltage collapse can be used as second measured variable in order to describe the ignition. To this end, a signal proportional to the current is measured with the aid of the oscilloscope together with the voltage. Likewise plotted in FIG. 1 and FIGS. 2a and b is the current IL(t) that flows through the lamp and for which the displacement current has been compensated. The powerPL(t)=UL(t)IL(t)   (1)is calculated as the product of the lamp voltage UL(t) and current IL(t).
The time dependent energy is then calculated by integrating the power PL(t) for the time t:
                              E          ⁡                      (            t            )                          =                              ∫            0                          t              p                                ⁢                                    PL              ⁡                              (                                  t                  ′                                )                                      ⁢                          ⅆ                              t                ′                                                                        (        2        )            
Also plotted in FIG. 1 and FIGS. 2 a, b are the power PL(t) and the calculated time dependent energy E(t). It is to be seen that the energy E(t) remains at a constant value after the voltage collapse and when the current has returned to zero. Since, as a rule, the high voltage pulses decay within a specific time, the energy that is coupled in during this time becomes the ignition energy Ez.